Because of the toxicity of mercury, there is great concern over any discharge of mercury into the environment. Until relatively recently, chemical plants did not take the necessary precautions when disposing of mercury containing materials. As a result, the soils and waters surrounding these plants are often contaminated with mercury or mercury compounds. The process of cleaning soils contaminated with metals is difficult since metals are strongly adsorbed or even diffused inside the pores of the soil matrix. Additionally, metallic mercury and most mercury species are not soluble in water.
If mercury exists in the soils in the presence of other metals, the removal process is even more costly and complicated. The price of disposing contaminated soils has increased dramatically in the past few years, particularly for disposal of mercury-contaminated soils, due to local, state and federal environmental regulations which restrict such disposal. Today, agencies that administer sites will usually require that the contaminated soils be treated rather than disposed of In most cases, more than one process technology is required to clean up the soils to meet federal and local standards, especially if more than one metal is present in the soils. This makes the clean up of the site more difficult, time consuming and expensive.
Several methods have been developed to recover the mercury from the solid materials. U.S. Pat. Nos. 5,314,527, 5,013,358 and 3,755,110 disclose processes that remove mercury from muds, sludge or other materials containing mercury compounds. However, most of these methods are complex and/or inefficient. Some even resulted in the discharge of a dangerous mercury vapor into the environment.
Leaching is most effectively accomplished when the solid particles are suspended in a liquid. However, heavy particles are generally difficult to suspend because they tend to settle out against the fluid motion of the liquid. As a result, when using agitation vessels, the heavy particles will accumulate and cause the system to clog or fail. Fractionation of the soil matrix may be used to avoid this problem but it complicates the process and increases the amount of time necessary to complete the cleaning process.
It is therefore the object of this invention to provide a process for cleaning mercury-contaminated soils that does not require fractionation of the soil matrix.
Another object of the invention is to provide a safe and efficient process for cleaning the mercury-contaminated water using sodium sulfide salts.
A further object of the invention is to provide an easy, economical and efficient process that can be used to clean soils contaminated with additional metals, such as zinc, copper and lead.